It has long been recognized that certain chemical compounds will absorb light energy (electromagnetic radiation) at particular frequencies and remit this absorbed energy at lower frequencies. Natural materials such as quinine and various minerals will absorb ultraviolet (UV) light and remit that energy as visible spectrum light, i.e., they fluoresce. The theory of fluorescence is discussed in Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, Volume 9, pages 485-491 (1966). Fluorescence is not limited to UV absorption and remission. So-called daylight fluorescent pigments will fluoresce when excited by radiation in both the UV and visible spectrum ranges. In those pigments the emitted fluorescence combines additively with the normal reflected color to the extent that these pigments appear to glow in normal daylight.
Chemical compounds which exhibit photochromism are also known. Photochromism is also a phenomenon which is well known and thoroughly documented. It is a reversible phenomenon in which exposure of certain compounds to electromagnetic radiation of a particular wavelength will cause a color change. When the radiation is removed, the original color will reappear. Although many photochromic compounds have been synthesized, most do not have a strong exhibition of color when in the non-excited state. This lack of color absent excitation energy has inhibited widespread use of photochromic compounds in commerce.